There is widely known a robot movement path control technique, as a realtime tracking technique using a laser sensor, in which the robot movement path is controlled by correcting a taught path according to the detection results of the laser sensor mounted on a robot hand for previously sensing the forward region in the advancing direction of the robot from moment to moment during a playback operation.
The realtime tracking technique using the laser sensor enables the robot to trace a proper movement path even in the case where individual workpieces are inaccurately positioned, and thus provides an extremely useful technical means for industrial robots for performing a variety of tasks.
However, when applying this technique to a welding robot for arc welding, there arises a problem of how to deal with a so-called tack welded portion. FIGS. 1, 2a and 2b illustrate this problem, wherein FIG. 1 shows an external appearance of a fillet weld joint having tack welded portions, FIG. 2a is a sectional view showing a position of a detected point by a laser sensor on a welding corner line of the fillet weld joint in a section without tack welded portion as shown in FIG. 1, and FIG. 2b is a sectional view showing a position of a detected point by the laser sensor on the welding corner line in a section with the tack welded portion. It is to be noted that a term "welding reference line" used in the following description denotes a line connecting reference points at which the welding is aimed (each reference point being a point to which a distal end of a welding wire is to be brought).
In FIGS. 1, 2a and 2b, symbols W1 and W2 represent two platelike workpieces forming a joint, and the welding is performed on a corner line formed between the two workpieces W1 and W2. Points P.sub.1, P.sub.2, . . . , P.sub.6 represent welding sections, wherein P.sub.1 is a welding start point (near a taught point T.sub.1) and P.sub.6 is a welding end point (near a taught point T.sub.2). In this example, a welding reference line WL forms a substantially straight path (not necessarily straight in the strict sense) indicated by P.sub.1 .fwdarw.P.sub.2 .fwdarw. . . . .fwdarw.P.sub.6.
A path of movement of the distal end point of a tool of the welding robot is desired to be coincide with the welding reference line WL (no shift for adjustment) or a line obtained by shifting the welding reference line by a predetermined amount for adjustment (e.g., a path spaced by 2 mm in the vertical or horizontal direction from the corner line).
In the case where some tack welded portions, such as A1 and A2 in FIG. 1, exist along the welding reference line WL, however, the corner line (the welding reference line WL) are covered and concealed with the tack welds in a section P.sub.2 -P.sub.3 and a section P.sub.4 -P.sub.5, and thus the position of the welding reference line WL cannot be detected by the laser sensor, making it difficult to define the desired path as mentioned above.
In other words, when scanning the corner line transversely with a laser beam LB from the laser sensor LS (with a deflection range SC) so as to detect the position of the corner line (the position of the welding reference line WL) as shown in FIG. 2a, a detected point SA on the corner line can be obtained in the sections (P.sub.1 -P.sub.2, P.sub.3 -P.sub.4, P.sub.5 -P.sub.6) where no tack welded portion exists. However, in the sections (2-P.sub.3, P.sub.4 -P.sub.5) where the tack welded portions exist, the detected point SA is obtained near an edge of the tack welded portion A, and thus is deviated from the position of the corner line (the welding reference line WL), as shown in FIG. 2b. This causes the robot path to be deflected laterally and deviated from the aforementioned desired path.
Further, depending on the shape or size of the tack welded portion A, there is found no detected point (detection failure) or the position of the detected point is disordered (erroneous detection). If such detection failure or erroneous detection occurs during an actual welding operation, the robot strays or suddenly stops in the vicinity of the tack welded portion to impede the welding operation.